The invention relates to a hybrid drive for an electric vehicle having a fuel cell and an energy store.
U.S. Pat. No. 5,334,463 A1 discloses a hybrid drive, comprising a fuel cell, a battery, an electric traction motor and electrical auxiliary loads, the fuel cell and the battery being connected in series with the electric traction motor and the electrical auxiliary loads by means of a common circuit, With sufficient voltage, the battery can be charged by the fuel cell.
Such an arrangement having only one circuit has the disadvantage that the larger of the two voltage sources, that is to say fuel cell or battery, determines the maximum driving power, since it is not possible to connect two current sources having different voltages to one load. This means that, at full load, the electric traction motor is supplied by the fuel cell or by the battery, depending on which of the two voltage sources provides greater power.
In addition, U.S. Pat. No. 5,519,312 A1 discloses a hybrid system, comprising a fuel cell and a superconductive magnetic store (SMES) which are connected to an electrical load by means of a converter. Suitable switches and a control unit can be used to connect the fuel cell to the SMES store and to connect the fuel cell and/or the SMES store to the electrical load.
A further hybrid system, comprising a fuel cell and two batteries connected in parallel therewith, is disclosed in EP 0 136 187 A2. Suitable switches can be used to isolate and connect the fuel cell and the batteries from/to the circuit.
Finally, U.S. Pat. No. 5,166,584 A1 discloses an electric vehicle having a hybrid system which comprises a battery and an electrical generator and in which, in overload mode, the current supply for auxiliary loads is reduced in order to ensure sufficient current supply for the traction drive.
It is the object of the invention to provide a hybrid drive for electric vehicles having improved driving powers.
The object is achieved by the characterizing features of Patent claim 1.
The hybrid drive according to the invention has the advantage that it permits variable control, which, in particular, improves the maximum driving power. In the first illustrative embodiment, the auxiliary loads can be supplied by the energy store at full load. This means that the maximum fuel cell power is available for driving the vehicle. In all the other illustrative embodiments, the electric traction motor can be driven by the fuel cell and the energy store at full load, which means that the maximum driving power is given as the sum of fuel cell and energy store power.
In addition, in braking mode, the electrical auxiliary loads can be supplied by the fuel cell, which means that the energy store can also be charged at low brake powers, that is to say even below the power requirement of the electrical auxiliary loads.
The use of a DC/DC converter in the connecting line between the two circuits also allows the energy store to be charged when the vehicle is at a standstill, irrespective of the voltage or the load state of the fuel cell. In addition, the energy store can also be charged and discharged while the vehicle is in motion, irrespective of the voltage or the load state of the fuel cell. Nevertheless, undesirable losses through the DC/DC converter arise only when the energy store is being charged or when the electric traction motor is being operated by the energy store. Since the fuel cell is connected to the electric traction motor without the interposition of a DC/DC converter, no such losses arise when the electric traction motor is being operated by the fuel cell.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from a reading of the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings.